The invention relates to propylene polymers containing a matrix of a propylene homopolymer and a copolymer of propylene and other alkenes, wherein, during the separation of the propylene polymers according to tacticity and comonomer distribution of the polymer chains, by first dissolving the propylene polymers in boiling xylene, then cooling the solution at a cooling rate of 10xc2x0 C./h to 25xc2x0 C. and thereafter, increasing the temperature, separating the propylene polymers into fractions of different solubility, either one or more of the conditions that
i) more than 20% by weight of the matrix remain undissolved on further heating to 112xc2x0 C. or
ii) more than 8% by weight of the matrix remain undissolved on further heating to 117xc2x0 C. or
iii) more than 1% by weight of the matrix remain undissolved on further heating to 122xc2x0 C.
are fulfilled by the matrix which remains undissolved on heating the cooled propylene polymer solution to 80xc2x0 C.
The present invention furthermore relates to a process for the preparation of the propylene polymers, their use for the production of films, fibers or moldings and films, fibers or moldings comprising these propylene polymers.
The unpublished German patent application P 197 10 761.3 describes propylene homopolymers which have particularly high crystallinity and are prepared using a catalyst system containing a titanium-containing solid component, an aluminum compound and an electron donor compound.
Propylene/ethylene copolymers obtainable by polymerization over Ziegler-Natta catalysts have been described in many patents. U.S. Pat. No. 4,260,710 discloses the preparation of homo- and copolymers of alk-1-enes by polymerization with the aid of Ziegler-Natta catalysts in a stirred kettle. The catalyst components used contain, inter alia, compounds of polyvalent titanium, aluminum halides and/or alkylaluminum as well as electron donor compounds, silanes, esters, ethers, ketones or lactones generally being used (EP-B 14 523, EP-B 45 977, EP-B 86 473, U.S. Pat. No. 857,613, EP-A 171 200).
Furthermore, a number of processes for the preparation of propylene/ethylene block copolymers with the aid of a Ziegler-Natta catalyst system are known (U.S. Pat. No. 4,454,299, U.S. Pat. No. 4,455,405, ZA-B 0084/3561, ZA-B 0084/3563, ZA-B 0084/5261, GB-B 1 032 945, DE-A 38 27 565), in which first gaseous propylene is polymerized in a first reaction zone and the homopolymer obtainable therefrom is then brought into a second reaction zone where a mixture of ethylene and propylene is polymerized thereon. The process is usually carried out at superatmospheric pressure and in the presence of hydrogen as a molecular weight regulator. The copolymers obtainable generally have good impact strength and rigidity.
By varying the amount of the homopolymer obtained in the first stage and of the copolymer to be polymerized on in the second stage, it is possible to control the impact strength of the propylene/ethylene block copolymers. However, inevitably a decrease in the rigidity is associated with an increase in the impact strength and, conversely, a decrease in the impact strength with an increase in the rigidity. For some applications of plastics, however, it is necessary to increase the rigidity without reducing the impact strength or to improve the impact strength without reducing the rigidity. Moreover, the polymers should have a very low chlorine content and, for economic reasons, a further increase in the productivity of the catalyst system used is of interest.
It is an object of the present invention to provide propylene polymers which have a further improved ratio of rigidity to toughness, possess a low chlorine content and furthermore can be prepared by a process with increased productivity.
We have found that this object is achieved by the propylene polymers defined at the outset and a process for their preparation, their use for the production of films, fibers or moldings and the films, fibers or moldings comprising these propylene polymers.
The novel propylene polymers are generally present in a form comprising at least two phases. They contain a continuous phase of a propylene homopolymer, which is referred to as the matrix. This matrix phase is semicrystalline, ie. it consists of a crystallized fraction, ie. the crystallites, and an amorphous fraction. According to the invention, the term propylene homopolymer is also intended to mean those propylene polymers in which minor amounts of other alkenes are copolymerized units, the propylene homopolymers generally containing less than 2, in particular less than 1, % by weight of other alkenes. The matrix of the novel propylene homopolymers preferably contains only propylene as a monomer.
The novel propylene polymers preferably contain from 50 to 92% by weight of the matrix-forming propylene homopolymer. Particularly preferably, the amount of the matrix is from 60 to 90% by weight.
Furthermore, the novel propylene polymers contain a copolymer of propylene and other alkenes, which is present as a separate phase from the matrix and exhibits very low or no crystallinity. This copolymer referred to as a rubber or elastomer contains, as a rule, from 20 to 85% by weight of propylene. The propylene content of the copolymer is preferably from 35 to 80, in particular from 45 to 70, % by weight. Depending on the chosen ratio of matrix to elastomer, the phase of the copolymer of propylene and other alkenes is present in disperse form in the matrix or is likewise continuous and hence cocontinuous with the matrix.
The other alkenes which are copolymerized with propylene are to be understood as meaning preferably alk-1-enes of up to 10 carbon atoms. Ethylene, but-1-ene, pent-1-ene, hex-1-ene, hept-1-ene and oct-1-ene or mixtures of these comonomers are particularly suitable, ethylene and but-1-ene being preferred. In the case of the copolymers acting as elastomers, unconjugated dienes, such as ethylidenenorbornene, dicyclopentadiene or 1,4-hexadiene, may also be present as other alkenes and are then copolymerized, as a rule, with ethylene and propylene.
The novel propylene polymers preferably contain from 8 to 50% by weight of the copolymer of propylene and other alkenes. Particularly preferably, the amount of the elastomer phase is from 10 to 40% by weight.
In addition to the essential components, the novel propylene polymers may also contain further homo- or copolymers of alkenes, for example homo- or copolymers of ethylene or polyisobutylene. For example, the amount of the further homo- or copolymers of alkenes may be from 0 to 30% by weight, based on the total propylene polymer.
One method for analyzing propylene polymers with regard to the fractions of polymer chains of different tacticity and different comonomer incorporation is TREF (Temperature Rising Elution Fractionation), in which the dissolution temperature of the polymer fraction corresponds to its average length of sequences built up without defects. To carry out the TREF, the propylene polymers are first dissolved in hot, preferably boiling xylene, the solution is then cooled at a constant cooling rate and thereafter the propylene polymers are separated, with increasing temperature, into fractions of different crystallinity. The composition of the propylene polymers can be described in terms of the fraction which remains undissolved on heating the cooled propylene polymer solution to a defined temperature.
In the fractionation of mixtures of propylene homopolymers and copolymers of the propylene with other alkenes, the polymer chains composed of different monomers first go into solution with increasing dissolution temperature, the dissolution temperature being lower the shorter the average sequence lengths of a monomer type. This means that first the elastomer fraction goes into solution and then, at higher temperatures, the matrix, which can be separated again with regard to the average isotactic sequence length of the polymer chains. It has been found that the polymer fractions which go into solution up to and including 80xc2x0 C. can be assigned to the elastomer phase, whereas the fraction which remains undissolved at 80xc2x0 C. and goes into solution only at higher temperatures originates from the matrix.
The novel propylene polymers have a high content of polymer chains with long defect-free isotactic propylene sequences in the matrix. Regarding the matrix, ie. that fraction of the propylene polymers which remains undissolved on heating the cooled polymer solution to 80xc2x0 C., the propylene polymers fulfill either one or preferably two or all of the following conditions i) to iii):
i) More than 20, preferably more than 30, in particular more than 40, % by weight of the matrix remain undissolved on further heating to 112xc2x0 C.
ii) More than 8, preferably more than 12, in particular more than 16, % by weight of the matrix remain undissolved on further heating to 117xc2x0 C.
iii) More than 1, preferably more than 2, in particular more than 3, % by weight of the matrix remain undissolved on further heating to 122xc2x0 C.
To determine according to the invention the composition of the propylene polymers, TREF is carried out in the following manner:
1-10 g of the propylene polymer are dissolved in a sufficient amount of boiling xylene to give a 0.5-2% strength by weight solution. The boiling solution is then cooled with a linear cooling rate of from 2 to 15, preferably from 10 to 25, xc2x0 C./h, or a lower temperature, the major part of the polymer being precipitated. The crystal suspension is then transferred to a thermostatable extraction apparatus which corresponds to that described by W. Holtup in Makromol. Chem. 178 (1977), 2355, and is heated to the first elution temperature. The polypropylene crystals are then extracted for at least 10 minutes at this temperature with vigorous mixing. Thereafter, the polymer solution is discharged while the polymer crystals remain behind in the extractor. The dissolved polymer is precipitated in cold acetone (temperature  less than 0xc2x0 C.), and the precipitate is filtered off and dried under reduced pressure until the weight is constant.
The extractor is then heated to the next elution temperature, and xylene at the same temperature is added in the same amount as that used for dissolution. Extraction is then carried out once again for at least 10 minutes with vigorous mixing, the polymer solution is discharged, the dissolved polymer is precipitated in cold acetone and the precipitate is filtered off and dried. These steps are repeated until the total polymer has dissolved.
The elastomer fraction corresponds to the fractions dissolved up to and including 80xc2x0 C., and the matrix fraction is the sum of all fractions which dissolve at higher elution temperatures. The matrix fraction insoluble at the individual temperatures is obtained from the ratio of the TREF fraction at the corresponding temperature of the total matrix fraction.
Since the matrix accounts for only a part of the novel propylene polymers, the fractions which remain undissolved on heating to 112xc2x0 C., 117xc2x0 C. and 122xc2x0 C. are, based on the total propylene polymer, naturally smaller and are dependent on the ratio of matrix fraction to elastomer fraction.
The novel propylene polymers can be prepared by mixing the melts of the fractions separately polymerized beforehand. Mixing is then effected in a manner known per se, for example by means of an extruder. The polymer blends prepared in such a manner may also contain polymers which are obtainable with other catalyst systems suitable for the preparation of the matrix, for example polyisobutylenes or ethylene/propylene/diene/(EPDM) rubbers.
However, the novel propylene polymers are preferably prepared in a multistage polymerization process, as a rule the matrix being polymerized in a first stage and a copolymer of propylene with other alkenes then being polymerized thereon in a second stage. In addition, it is possible in subsequent stages to polymerize on further homo- or copolymers of alkenes. Particularly suitable alkenes for such processes are alk-1-enes of up to 10 carbon atoms.
In the process likewise according to the invention, the propylene polymers are prepared by polymerization of propylene at from 50 to 120xc2x0 C. and from 15 to 40 bar in a first polymerization stage and subsequent copolymerization of propylene and other alk-1-enes of up to 10 carbon atoms at from 30 to 100xc2x0 C. and from 5 to 40 bar in a second polymerization stage in the presence of a Ziegler-Natta catalyst system which contains, as active components, a titanium-containing solid component a) which is obtained by reacting a titanium halide with a chlorine-free compound of magnesium, an inorganic oxide as carrier, a C1-C8-alkanol and an electron donor compound and, as cocatalysts, an aluminum compound b) and a further electron donor compound c).
For the preparation of the titanium-containing solid component a), the titanium halides used are the halides of trivalent or tetravalent titanium, for example TiBr3, TiBr4, TiCl3 or TiCl4, or alkoxytitanium halides, such as Ti(OCH3)Cl3, Ti(OC2H5)Cl3, Ti(O-iso-C3H7)Cl3, Ti(O-n-C4H9)Cl3, Ti(OC2H5)Br3, Ti(O-n-C4H9)Br3, Ti(OCH3)2Cl2, Ti(OC2H5)2Cl2, Ti(O-n-C4H9)2Cl2, Ti(OC2H5)2Br2, Ti(OCH3)3Cl, Ti(OC2H5)3Cl, Ti(O-n-C4H9)3Cl or Ti(OC2H5)3Br, the titanium halides which contain only halogen in addition to titanium and among these in particular the titanium chlorides and especially titanium tetrachloride, being preferred. According to the invention, the titanium halides may also be used as mixtures of one another or in mixtures with further titanium compounds. Examples of suitable further titanium compounds are titanium alcoholates, such as Ti(OCH3)4, Ti(OC2H5)4 or Ti(O-n-C4H9)4. Preferably, the titanium halides are used alone.
Furthermore, a chlorine-free compound of magnesium or a mixture of said compounds is used in the preparation of the titanium-containing solid component. Chlorine-free compounds of magnesium are to be understood according to the invention as meaning those which contain no halogen in their structural formula. However, the amounts of halogen contained as impurities in the novel chlorine-free compounds of magnesium should not exceed 5, in particular 2, % by weight. Suitable chlorine-free compounds of magnesium are in particular alkylmagnesiums and arylmagnesiums as well as alkoxymagnesium and aryloxymagnesium compounds, di-C1-C10-alkylmagnesium compounds preferably being used. Examples of novel chlorine-free compounds of magnesium are diethylmagnesium, di-n-propylmagnesium, di-isopropylmagnesium, di-n-butylmagnesium, di-sec-butylmagnesium, di-tert-butylmagnesium, diamylmagnesium, n-butylethylmagnesium, n-butyl-sec-butylmagnesium, n-butyloctylmagnesium, diphenylmagnesium, diethoxymagnesium, di-n-propyloxymagnesium, di-isopropyloxymagnesium, di-n-butyloxymagnesium, di-sec-butyloxymagnesium, di-tert-butyloxymagnesium, diamyloxymagnesium, n-butyloxyethoxymagnesium, n-butyloxy-sec-butyloxymagnesium, n-butyloxyoctyloxymagnesium and diphenoxymagnesium. Among these, n-butylethylmagnesium and n-butyloctylmagnesium are particularly preferred.
In addition, the titanium-containing solid component a) contains at least one inorganic oxide as carrier. As a rule, a finely divided inorganic oxide which has a median particle diameter from 5 to 200 xcexcm, preferably from 15 to 100 xcexcm and in particular from 20 to 70 xcexcm, is used as carrier. The median particle diameter is to be understood here as meaning the volume-related median value of the particle size distribution determined by Coulter Counter Analysis according to ASTM Standard D 4438.
Preferably, the particles of the finely divided inorganic oxide are composed of primary particles which have a median diameter of the primary particles of from 1 to 20 xcexcm, in particular from 3 to 10 xcexcm. The primary particles are porous, granular oxide particles which in general are obtained by milling a hydrogel of the inorganic oxide. It is also possible to sieve the primary particles before their further processing.
Furthermore, the inorganic oxide preferably to be used is also characterized by the fact that it has cavities or channels with an average diameter of from 0.1 to 20 xcexcm, in particular from 1 to 15 xcexcm, whose macroscopic volume fraction is from 5 to 30%, in particular from 10 to 30%, based on the total particle.
The median diameter of the primary particles and the macroscopic volume fraction of the cavities and channels of the inorganic oxide are advantageously determined by image analysis with the aid of scanning electron microscopy or electron probe microanalysis, in each case on particle surfaces and on particle cross-sections of the inorganic oxide. The electron micrographs obtained are evaluated and the median diameter of the primary particles and the macroscopic volume fraction of the cavities and channels are determined therefrom. The image analysis is preferably carried out by conversion of the electron microscopy data into a gray scale binary image and digital evaluation by means of a suitable computer program.
The inorganic oxide preferably to be used may be retained, for example, by spray-drying the milled hydrogel, which is mixed with water or an aliphatic alcohol for this purpose. Such finely divided inorganic oxides are also commercially available.
Finely divided inorganic oxide further usually has a pore volume of from 0.1 to 10, preferably 1.0 to 4.0, cm3/g and a specific surface area of from 10 to 1000, preferably from 100 to 500, m2/g, these being understood as the values determined by mercury porosymmetry according to DIN 66133 and by nitrogen adsorption according to DIN 66131.
The pH of the inorganic oxide, ie. the negative logarithm to the base 10 of the proton concentration, is preferably from 1 to 6.5, in particular from 2 to 6, particularly preferably from 3.5 to 5.5.
Particularly suitable inorganic oxides are the oxides of silicon, of aluminum, of titanium or of one of the metals of main groups I and II of the Periodic Table. Silica (silica gel) is a very preferably used oxide in addition to alumina or magnesium oxide or a sheet silicate. Mixed oxides, such as aluminum silicates or magnesium silicates, may also be used.
The inorganic oxides used as carrier contain hydroxyl groups on their surface. By elimination of water, it is possible to reduce or completely eliminate the content of OH groups. This can be achieved by thermal or chemical treatment. Thermal treatment is usually carried out by heating the inorganic oxide for from 1 to 24, preferably from 2 to 20, in particular from 3 to 12, hours to 250 to 900xc2x0 C., preferably 600 to 800xc2x0 C. The hydroxyl groups may also be removed by chemical methods, by treating the inorganic oxides with conventional drying agents, such as SiCl4, chlorosilanes or alkylaluminums. Preferably used inorganic oxides contain from 0.5 to 5% by weight of water. The water content is conventionally determined by drying the inorganic oxide at 160xc2x0 C. under atmospheric pressure until the weight is constant. The weight decrease corresponds to the original water content.
In the preparation of the titanium-containing solid component a), preferably from 0.1 to 1.0, in particular from 0.2 to 0.5, mol of the chlorine-free compound of magnesium is used per mole of the inorganic oxide.
Furthermore, C1-C8 alkanols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, n-hexanol, n-heptanol, n-octanol or 2-ethylhexanol or mixtures thereof, are used in the preparation of the titanium-containing solid component a). Ethanol is preferably used.
The titanium-containing solid component a) furthermore contains electron donor compounds, for example mono- or polyfunctional carboxylic acids, carboxylic anhydrides or carboxylic esters, as well as ketones, ethers, alcohols, lactones or organophosphorus or organosilicon compounds.
Preferably used electron donor compounds within the titanium-containing solid component are carboxylic acid derivatives and in particular phthalic acid derivatives of the formula (II) 
where X and Y are each chlorine, bromine or C1-C10-alkoxy or together are oxygen in an anhydride function. Particularly preferred electron donor compounds are phthalic esters, X and Y being C1-C8-alkoxy, for example methoxy, ethoxy, n-propyloxy, isopropyloxy, n-butyloxy, sec-butyloxy, isobutyloxy or tert-butyloxy. Examples of preferably used phthalic esters are diethyl phthalate, di-n-butyl phthalate, di-isobutyl phthalate, dipentyl phthalate, dihexyl phthalate, di-n-heptyl phthalate, di-n-octyl phthalate and di-2-ethylhexyl phthalate.
Other preferred electron donor compounds within the titanium-containing solid component are diesters of 3- or 4-membered, unsubstituted or substituted cycloalkyl-1,2-dicarboxylic acids, and monoesters of substituted benzophenone-2-carboxylic acids or substituted benzophenone-2-carboxylic acids. Hydroxy compounds used in these esters are the alkanols usually employed in esterification reactions, for example C1-C15-alkanols or C5-C7-cycloalkanols, which in turn may carry one or more C1-C10-alkyl groups, and furthermore C6-C10-phenols.
Mixtures of different electron donor compounds may also be used.
The titanium-containing solid component a) can be prepared by methods known per se which are described, for example, in EP-A 171 200, GB-A 2 111 066, U.S. Pat. No. 4,857,613 and U.S. Pat. No. 5,288,824. However, in the first stage of its preparation, the chlorine-free compound of magnesium in an inert solvent is used and is reacted with the carrier and C1-C8-alkanol to give a chlorine-free intermediate which is advantageously further processed without purification or extraction.
In the preparation of the titanium-containing solid component a), the following two-stage process is preferably used:
In the first stage, a solution of the chlorine-free compound of magnesium is first added to the inorganic oxide in an inert solvent, preferably a liquid alkane or an aromatic hydrocarbon, eg. toluene or ethylbenzene, after which this mixture is allowed to react for from 0.5 to 5 hours at from 10 to 120xc2x0 C., as a rule with stirring. The C1-C8-alkanol is then added in at least a 1.3-fold, preferably from 1.6- to 3-fold, in particular from 1.8- to 2.0-fold, molar excess, based on the magnesium-containing compound, usually with continuous stirring at from xe2x88x9220 to 80xc2x0 C., preferably from 0 to 40xc2x0 C. This results in the formation of a chlorine-free intermediate, which is preferably further processed without purification or extraction. After from about 10 to 120, preferably from about 20 to 60, minutes, the titanium halide and the electron donor compound are added to this intermediate at from 10 to 50xc2x0 C. From 1 to 15, preferably 2 to 5, mol of the titanium halide and from 0.01 to 1, in particular from 0.3 to 0.7, mol of the electron donor compound are used per mole of magnesium of the solid obtained from the first stage. This mixture is allowed to react for at least 10, preferably 30, in particular from 45 to 90, minutes at from 10 to 150xc2x0 C., in particular from 60 to 130xc2x0 C., in general with stirring, and the solid substance thus obtained is then filtered off and washed with a C7-C10-alkylbenzene, preferably with ethylbenzene.
In the second stage, the solid obtained from the first stage is extracted at from 100 to 150xc2x0 C. with excess titanium tetrachloride or with a solution of titanium tetrachloride in an inert solvent, preferably a C7-C10-alkylbenzene, which solution is present in excess, the solvent containing at least 5% by weight of titanium tetrachloride. The extraction is carried out as a rule for at least 30 minutes. The product is then washed with a liquid alkane until the titanium tetrachloride content of the wash liquid is less than 2% by weight.
The titanium-containing solid component a) obtainable in this manner is used with cocatalysts as the Ziegler-Natta catalyst system. Suitable cocatalysts are aluminum compounds b) and further electron donor compounds c).
Examples of suitable aluminum compounds b) are trialkylaluminum, and those compounds in which an alkyl group has been replaced by an alkoxy group or by a halogen atom, for example by chlorine or bromine. The alkyl groups may be identical or different. Linear or branched alkyl groups are suitable. Preferably used trialkylaluminum compounds are those whose alkyl groups are each of 1 to 8 carbon atoms, for example trimethylaluminum, triethylaluminum, triisobutylaluminum, trioctylaluminum or methyldiethylaluminum or mixtures thereof.
In addition to the aluminum compound b), electron donor compounds c), for example mono- or polyfunctional carboxylic acids, carboxylic anhydrides and carboxylic esters, as well as ketones, ethers, alcohols, lactones and organophosphorus and organosilicon compounds are used as the further cocatalyst, it being possible for the electron donor compounds c) to be identical to or different from the electron donor compounds used for the preparation of the titanium-containing solid component a). Preferred electron donor compounds are organosilicon compounds of the formula (I)
R1nSi(OR2)4-nxe2x80x83xe2x80x83(I) 
where the radicals R1 are identical or different and are each C1-C20-alkyl, 5- to 7-membered cycloalkyl which in turn may be substituted by C1-C10-alkyl, C6-C18-aryl or C6-C18-aryl-C1-C10-alkyl, the radicals R2 are identical or different and are each C1-C20-alkyl and n is the integer 1, 2 or 3. Particularly preferred compounds are those in which R1 is C1-C8-alkyl or 5- to 7-membered cycloalkyl, R2 is C1-C4-alkyl and n is 1 or 2.
Among these compounds, dimethoxydiisopropylsilane dimethoxyisobutylisopropylsilane, dimethoxydiisobutylsilane, dimethoxydicyclopentylsilane, dimethoxydicyclohexylsilane, dimethoxycyclohexylmethylsilane, dimethoxyisopropyl-tert-butylsilane, dimethoxyisobutyl-sec-butylsilane and dimethoxyisopropyl-sec-butylsilane are particularly noteworthy.
The compounds b) and c) acting as cocatalysts may be allowed to act individually, in succession in any desired order or together as a mixture on the titanium-containing solid component a). This is usually effected at from 0 to 150xc2x0 C., in particular from 20 to 90xc2x0 C., and from 1 to 100, in particular from 1 to 40 bar.
The cocatalysts b) are preferably used in such an amount that the atomic ratio of aluminum from the aluminum compound b) to titanium from the titanium-containing solid component a) is from 10:1 to 800:1, in particular from 20:1 to 200:1.
In the process leading to the novel propylene polymers and likewise according to the invention, the molar ratio of the aluminum compound b) to the further electron donor compound c) is adjusted to from 1.5:1 to 9:1, in particular 2:1 to 8:1. If a molar ratio greater than 9:1 is used, the stereospecificity of the resulting propylene homopolymers is in general insufficient. If the ratio is less than 1.5:1, the process is generally not very advantageous for economic reasons, in particular owing to the large amounts of further electron donor compound c) then used.
Said process can be carried out in the conventional reactors used for the polymerization of C2-C10-alk-1-enes, either batchwise or, preferably, continuously, inter alia in solution, as suspension polymerization or, preferably, as gas-phase polymerization. Suitable reactors include continuously operated stirred reactors, loop reactors or fluidized-bed reactors. If a batchwise procedure is used, polymerization is effected in the stages in succession by varying the reaction conditions. As a rule, however, the continuous procedure is used, the matrix initially being prepared in the first stage in one or more reactors and then the elastomer phase being prepared in one or more further reactors. In addition, it is possible to polymerize on further homo- or copolymers of C2-C10-alk-1-enes in subsequent stages. It is also possible to combine different reactor types. In a particularly preferred embodiment, the preparation of the copolymers is carried out in a multistage, in particular two-stage, gas-phase process, the reactors used being continuously operated stirred kettles which contain a fixed bed of finely divided polymer which is usually kept in motion by suitable stirring apparatuses.
Said process is carried out by polymerizing propylene or a monomer mixture comprising propylene and minor amounts of other alk-1-enes of up to 10 carbon atoms at from 50 to 120xc2x0 C., preferably from 60 to 90xc2x0 C., and from 15 to 40, preferably from 20 to 35, bar for the preparation of the matrix in the first polymerization stage. The average residence time is as a rule from 1.0 to 3.0, preferably from 0.5 to 5, hours. The reaction conditions are preferably chosen so that, in the first polymerization stage, from 0.05 to 2, preferably from 0.1 to 1.5 kg of the matrix are formed per mole of the aluminum component b). Polymerization is usually carried out in the presence of hydrogen, a partial pressure ratio of propylene to hydrogen of, preferably, from 30:1 to 180:1, in particular from 30:1 to 150:1, being established.
The propylene polymer obtained is discharged together with the catalyst from the first polymerization stage and is introduced into the second polymerization stage, where a copolymer of propylene and other alk-1-enes of up to 10 carbon atoms is polymerized onto said propylene polymer. The amount of added monomers is as a rule such that the ratio of the partial pressure of propylene to that of the other alk-1-enes of up to 10 carbon atoms is from 0.5:1 to 20:1, in particular from 1.0:1 to 10:1. Moreover, by a suitable choice of the reaction parameters, the weight ratio of the monomers reacted in the first polymerization stage to those reacted in the second polymerization stage is usually established to be from 0.5:1 to 20:1, in particular from 1.0:1 to 15:1.
The polymerization is effected in the second polymerization stage at from 30 to 100xc2x0 C., preferably from 50 to 90xc2x0 C., and from 5 to 40, preferably from 10 to 35, bar. The average residence time of the reaction mixture is as a rule from 0.5 to 5, preferably 1.0 to 3.0, hours. The pressure in the second polymerizaton stage is preferably at least 3 bar below the pressure of the first polymerization stage. In the second polymerization stage, too, the polymerization is preferably carried out in the presence of hydrogen, the ratio of the partial pressure of propylene to that of hydrogen being as a rule from 100:1 to 5:1, in particular from 60:1 to 10:1.
It may also be advisable to add a C1-C8-alkanol, in particular a C1-C4-alkanol, to the reaction mixture of the second polymerization stage, which alkanol influences the activity of the Ziegler-Natta catalyst. Alkanols suitable for this purpose are, for example, methanol, ethanol, n-propanol, n-butanol and very particularly isopropanol. The amount of C1-C8-alkanol is in general such that the molar ratio of the C1-C8-alkanol to the aluminum compound b) serving as cocatalyst is from 0.01:1 to 10:1, in particular from 0.02:1 to 5:1.
The weight average molar masses of the novel copolymers of propylene are as a rule from 10,000 to 1,000,000 g/mol and the melt flow rates (MFR) are from 0.1 to 100, preferably from 0.2 to 20, g/10 min. The melt flow rate (MFR) corresponds to the amount of polymer which is forced, in the course of 10 minutes at 230xc2x0 C. and under a weight of 2.16 kg, out of the test apparatus standardized according to ISO 1133.
Compared with the propylene polymers known to date, the novel propylene polymers have a higher proportion of polymer chains with long defect-free isotactic polymer sequences in the matrix. This manifests itself, for example, in an improved ratio of rigidity to toughness. Moreover, the novel copolymers also have reduced chlorine contents. The productivity of the process used for the preparation of these copolymers is substantially higher compared with the known processes.
Owing to their good mechanical properties, the novel propylene polymers are particularly suitable for the production of films, fibers or moldings.